At the top of the Firefox window, click the Firefox button, go over to the History menu and select Clear Recent History...
For Windows XP: At the top of the Firefox window, click the Tools menu and select Clear Recent History...

Select how much history you want to clear: â—¦ Click the drop-down menu next to Time range to clear to choose how much of your history Firefox will clear.
 â—¦Next, click the arrow next to Details to select exactly what information will get cleared. Your choices are described in the What things are included in my history? section above.

Finally, click the Clear Now button and the window will close and the items you've selected will be cleared.